[NAIROBI] An invasive weed may worsen the malaria epidemic in East Africa by sustaining the mosquitoes that transmit the disease, a study says.

Published in PLOS One last month (14 September), the paper shows that an aggressive weed called Parthenium hysterophorus provides mosquitoes with enough energy to live longer than they would from feeding on two other existing weeds. This could increase the mosquitoes’ chances of multiplying and transmitting malaria parasites, the study says.

Researchers fed female Anopheles gambiae mosquitoes on nectar from P. hysterophorus and two other weeds that are abundant in malaria endemic regions in western Kenya and that insects could feed on: Ricinus communis and Bidens pilosa.

They found that both P. hysterophorus and R. communis gave the mosquitoes enough energy, in the form of sugar and fat reserves, to enable the insects to survive several days longer than those that fed on B. pilosa. But while the mosquitoes were able to withstand the toxins released by P. hysterophorus, which are poisonous to livestock and human beings, the R. communis toxin killed them.

Baldwyn Torto, a scientist from Icipe, the International Centre of Insect Physiology and Ecology in Kenya, and author of the study, says the nectar’s sugar provides energy for mosquitoes to survive. But to lay eggs and reproduce, the female mosquitoes need to bite animals or people to drink their blood.

Removing the source of nectar would mean that mosquitoes could die before they get blood, cutting the transmission risk. “The problem with this weed is that it can also thrive when there’s less rainfall, so it’s dangerous,” Torto says.

The weed is aggressive and can even displace other sources of nectar, such as R. communis, Torto warns. “Awareness is needed in malaria hotspots,” he says. “When they spot it, communities should uproot the weed before it flowers and multiplies.”

He adds that, in most parts of the world, people are concerned about the effect of invasive weeds on agriculture but rarely focus on their health impacts.

Simon Kariuki, a malaria researcher at the Kenya Medical Research Institute, says the study does not go far enough to draw conclusions about the Parthenium weed’s actual effect on malaria transmission.

“After the mosquitoes lived for longer, were they able to lay eggs, multiply and transmit malaria? That is not shown in the study,” Kariuki says. The paper also does not show how fast the weed spreads or how much impact it will have in areas where other control strategies are in place to fight malaria, he adds.

Njagi Kiambo, a malaria expert at Kenya’s ministry of health, says that using plants to control malaria can be a useful intervention. “But nature is unpredictable and hard to control,” he says, “so pesticides — used in bednets and residual sprays — are given priority as their impact is immediate and reliable.”